1. Field of Invention
The invention relates to a process for the production of a multi-layer coating based on a base coat layer and a UV curable clear coat layer and in particular is used for coating vehicles and for the repair of vehicle coatings.
2. Description of Related Art
In vehicle repair coating, apart from coating entire vehicle bodies and large body parts, touching up relatively small blemished areas by means of spot repair is an efficient and economic alternative. To improve the efficiency of the repair coating of small blemished areas and bodywork parts, endeavors are being made to reduce conventional drying or curing times.
It is known that the various coating layers of a multi-layer structure, such as, for example, the filler, base coat, clear coat and/or one-layer top coat layer, may be cured extremely rapidly by means of UV (ultraviolet light) radiation if an appropriate binder is used. Even by using just one UV curable coating layer in the multi-layer structure, for example, a UV curable clear coat layer makes a considerable contribution to reducing drying and curing times and thus reduces the overall processing time. Solvent-containing or solvent-free UV curable clear coats are generally based on polymeric and/or oligomeric binders in combination with low molecular weight UV curable reactive diluents having appropriate functional groups for UV curing. However, if such clear coats containing reactive diluents are applied onto solvent-based or water-based base coat layers and cured by means of UV radiation, they often result in severe solvent attack of the underlying base coat layer and sometimes cause considerable, unacceptable changes to the colour of the base coat layer. In extreme cases, the base coat/clear coat structure may even be completely destabilised, which may result in the entire coating structure becoming detached from the substrate.
EP 568967 describes a process, in particular for use in vehicle original coating, in which a thermally curable clear coat layer is first applied onto a pigmented base coat layer and then the base coat and clear coat layer are stoved (baked) at temperatures of up to 150° C. A radiation-curable clear coat layer is then applied and cured.
DE 199 20 801 describes a process in which a thermally curable and radiation-curable clear coat is first applied onto a base coat layer and partially cured. Then a second radiation curable clear coat containing nanoparticles is applied and both clear coats are cured thermally and by irradiation with UV radiation.
The problems which occur in the multi-layer structure when reactive diluents are used in UV curable clear coats are not addressed in these references.
In order to permit the use of a multi-layer coating structure comprising a pigmented base coat and a UV curable clear coat containing reactive diluents, in particular also for vehicle repair coating, it is necessary to prevent or at least greatly reduce any colour change and destabilisation of the base coat layer or of the base coat/clear coat structure. There is accordingly a requirement for a suitable process which makes it possible in the above multi-layer structure to use UV curable clear coats based on low molecular weight reactive diluents without encountering the above disadvantages of the prior art.